Conventionally, various CAD tools, automatic design methods, etc. have been proposed on the mask pattern design of semiconductor integrated circuits. Especially, the design using an automatic lay-out tool is a main stream in the mask pattern design of semiconductor integrated circuits which are applied to a specific use called "ASIC", because the semiconductor integrated circuits are required to be delivered to customers in a short term.
A conventional automatic design process includes steps of sampling coordinates of the most appropriate positions for pads, determining an arrangement of pad blocks each including a pad, a transistor region and a logic circuit region, determining an arrangement of a pad, a transistor region, and a logic circuit region in each pad block, and interconnecting the pad, the transistor region, and the logic circuit region. In the pad block, the pad is connected to an inner lead of a lead frame, and to the transistor region including N and P channel transistors for providing an output buffer circuit which is driven by the logic circuit region, and the transistor and logic circuit regions are provided with power supply terminals and ground terminals.
In the coordinate sampling step, a coordinate of the most appropriate pad position is determined in consideration of a lead frame and a chip size. In the next step, a pad block is arranged, such that a center coordinate of a pad is coincided with the determined coordinate. Then, automatic arrangement and interconnection of the internal regions are carried out in each pad block in accordance with coordinates of a pad block which is arranged at the preceding step.
However, the conventional automatic design process has a disadvantage in that a pad can not be provided at a corner region of a semiconductor chip having an area smaller than that for accommodating a designed pad block, because a pad is designed to be positioned in a pad block. In such a case, a pad which is positioned in a pad block is manually shifted to the corner region of the semiconductor chip under the condition that the pad block is provided proximate to the corner region, and interconnection is completed among the internal regions of the pad block, and the shifted pad is manually connected to the original pad block.